How to get it right: Notches & holes in solid timber joists

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Notches and holes in solid timber joists

When installing pipework or cabling, floor or ceiling joists may need to be notched or drilled. But this could weaken the floor to such an extent that it becomes structurally unsound.

To find out if this might be the case, you can find guidance on notches and holes in solid timber joists for use in domestic properties in section 3.22 of Eurocode 5 published by BM TRADA.

The diagrams and tables below illustrate the zones and sizes permissible for differing spans and joist depths.

Notches and holes in sold timber joists - diagram 1Notches and holes in solid timber joists


Span of timber Hole locations between Notching locations between
mm 0.25 of span 0.4 of span 0.07 of span 0.25 of span
2000 500 800 140 500
2500 625 1000 175 625
3000 750 1200 210 750
3500 875 1400 245 875
4000 1000 1600 280 1000
4500 1125 1800 315 1125


Depth of timber Max hole diameter Max notch depth
mm 0.25 x depth mm 0.125 x depth mm
100 25.0 12.5
125 31.3 15.6
150 37.5 18.8
175 43.8 21.9
200 50.0 25.0
225 56.3 28.1
250 62.5 31.3
275 65 (max) 34.4
300 65 (max) 35

Key points for notches and holes

Before you start work check that the joist size is adequate for the span.

Notches can only be made in the top OR bottom of the joists, (not both) within the permitted area to a maximum of 35mm.

Holes may only be made on the joist's centreline within the permitted area to a maximum diameter of 65mm.

Additional holes must be at least three diameters (centre to centre) apart.

Single/multiple timber structural beams, rafters, purlins and binders must never be notched or drilled without calculations to justify the remaining timber. Rafters may be birdsmouthed by no more than 1/3rd of the depth.

This guidance does not apply to engineered joists e.g. JJI joists and reference should be made to specific product manufacturers instructions.

Further reading

How to get it right: Supporting joists on external walls

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.

This article was updated on 22 March 2022


drilling of joists

Submitted 3 years 6 months ago

check the hole locations
2m span .4 X span 880 not 8800


Submitted 3 years 6 months ago

Hello there

Thanks for the spot - we've amended that.

Julie, LABC

table correction

Submitted 3 years 5 months ago

2 metre span x .4 is 800mm not 880


Submitted 3 years 5 months ago

Done - thanks so much.

Calculating notch

Submitted 2 years 2 months ago

Hi, i got a plumbing processes exam and i recently got a question that says ' a customer joist has a span of 4m- how near to the supporting wall can the notch start'? I just been wondering how you meant to calculate this type of question the answer is 280mm.


Submitted 2 years 2 months ago


Thank you for your query.

As noted in the diagram and table above, which is taken from guidance published by BM TRADA, notches should not be cut closer to the support than 0.07 of the span.

Kind regards,
Dalila, LABC

Template for notches and holes in joists

Submitted 2 years ago

Have you seen a Template for notching and drilling holes in joists.

Floor joists

Submitted 1 year 3 months ago

A joist needs to be notched to lay some 22mm copper pipe.The existing joist is 15mm deep can this be achieved? Can you show me how to work it out

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